The several antigenic types of botulinum toxin are all 140,000 to 160,000 in mol wt and are synthesized as singlle chain proteins having only part of their potential toxicities. They are activated, naturally or experimentally, by proteases to full toxicity. During the enzymatic toxicity enhancement, the single chains are converted into dichains composed of a 100,000 mol wt polypeptide (H chain) linked by disulfide bond to a 50,000 mol wt subunit (L chain). Tetanus toxin is a very similar molecule; the intracellular, single chain protein of mol wt 160,000 is converted by proteases into a disulfide-linked dichain in which the larger chain is about twice the size of the smaller. Subunit chains of several botulinum toxin types will be purified. Complementary chains (e.g., H chain of type A toxin and L chain of type B toxin) will be mixed together in order to assemble toxic, hybrid dichains. Hybridization of complementary subunits of botulinum and tetanus toxins will be studied. Antitoxic potentials of antibody to L chain will be compared to that of anti-H and H and L chains will be studied for numbers of antigenic determinants. The peptide bond scissions responsible for enzymatic toxicity activations will be determind from comparisons of the terminal amino acids of purified, single chain type E toxin with those of molecules partially activated with clostripain and after complete activation with trypsin. Data obtained in this study will show if trypsin nicks the single chain molecule so that the original NH2-terminal becomes part of L or H chain.